server: add tiflash fallback testcase (#23248)

Co-authored-by: Yiding Cui <winoros@gmail.com>

server/conn_test.go

@@ -778,7 +778,11 @@ func (ts *ConnTestSuite) TestTiFlashFallback(c *C) {
 	c.Assert(cc.handleQuery(ctx, "select * from t t1 join t t2 on t1.a = t2.a"), NotNil)
 	c.Assert(failpoint.Disable("github.com/pingcap/tidb/server/secondNextErr"), IsNil)
 
-	// TODO: simple TiFlash query (unary + non-streaming)
+	// simple TiFlash query (unary + non-streaming)
+	tk.MustExec("set @@tidb_allow_batch_cop=0; set @@tidb_allow_mpp=0;")
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/tikvStoreSendReqResult", "return(\"requestTiFlashError\")"), IsNil)
+	testFallbackWork(c, tk, cc, "select sum(a) from t")
+	c.Assert(failpoint.Disable("github.com/pingcap/tidb/store/tikv/tikvStoreSendReqResult"), IsNil)
 
 	// TiFlash query based on batch cop (batch + streaming)
 	tk.MustExec("set @@tidb_allow_batch_cop=1; set @@tidb_allow_mpp=0;")

store/mockstore/unistore/rpc.go

@@ -223,11 +223,6 @@ func (c *RPCClient) SendRequest(ctx context.Context, addr string, req *tikvrpc.R
 	case tikvrpc.CmdRawScan:
 		resp.Resp, err = c.rawHandler.RawScan(ctx, req.RawScan())
 	case tikvrpc.CmdCop:
-		failpoint.Inject("copRpcErr"+addr, func(value failpoint.Value) {
-			if value.(string) == addr {
-				failpoint.Return(nil, errors.New("cop rpc error"))
-			}
-		})
 		resp.Resp, err = c.usSvr.Coprocessor(ctx, req.Cop())
 	case tikvrpc.CmdCopStream:
 		resp.Resp, err = c.handleCopStream(ctx, req.Cop())

store/tikv/region_request.go

@@ -254,6 +254,10 @@ func (s *RegionRequestSender) SendReqCtx(
 			if sType == kv.TiDB {
 				failpoint.Return(nil, nil, ErrTiKVServerTimeout)
 			}
+		case "requestTiFlashError":
+			if sType == kv.TiFlash {
+				failpoint.Return(nil, nil, ErrTiFlashServerTimeout)
+			}
 		}
 	})